v3rpc: add ErrConnClosed for closed client
v3rpc: add ErrConnClosed for closed client

For https://github.com/coreos/etcd/issues/5495.

wantonsolutions/dinv-etcd,lorneli/etcd,zaphod-concur/etcd,lorneli/etcd,chipironcin/etcd,heyitsanthony/etcd,coreos/etcd,ravigadde/etcd,plasticbox/etcd,xiang90/etcd,glevand/coreos--etcd,StorReduce/etcd,visheshnp/etcd,chipironcin/etcd,a-robinson/etcd,joshix/etcd,rnaveiras/etcd,ajityagaty/etcd,StorReduce/etcd,sebrandon1/etcd,yudai/etcd,rchicoli/etcd,OyTao/etcd_learning,li-ang/etcd,glycerine/etcd,mkumatag/etcd,bluebreezecf/etcd,rnaveiras/etcd,sofuture/etcd,mickep76/etcd,mkumatag/etcd,etcd-io/etcd,sinsharat/etcd,handsomesun/etcd,sinsharat/etcd,qqsun8819/etcd,dannysauer/etcd,purpleidea/etcd,endocode/etcd,swingbach/etcd,fanminshi/etcd,wantonsolutions/dinv-etcd,akauppi/etcd,bdarnell/etcd,YuleiXiao/etcd,sofuture/etcd,luxas/etcd,tguangch/etcd,luxas/etcd,armstrongli/etcd,dannysauer/etcd,rchicoli/etcd,CrazyJvm/etcd,fanminshi/etcd,vimalk78/etcd,gyuho/etcd,whuwxl/etcd,ravigadde/etcd,xiang90/etcd,plasticbox/etcd,armstrongli/etcd,cockroachdb/etcd,joshix/etcd,gyuho/etcd,mickep76/etcd,visheshnp/etcd,wantonsolutions/dinv-etcd,lumjjb/etcd,zaphod-concur/etcd,mqliang/etcd,ktateish/etcd,tguangch/etcd,yudai/etcd,qqsun8819/etcd,a-robinson/etcd,purpleidea/etcd,bdarnell/etcd,whuwxl/etcd,etcd-io/etcd,cockroachdb/etcd,coreos/etcd,lumjjb/etcd,mqliang/etcd,mkumatag/etcd,glevand/coreos--etcd,nekto0n/etcd,jlamillan/etcd,mqliang/etcd,bluebreezecf/etcd,endocode/etcd,OyTao/etcd_learning,insequent/etcd,heyitsanthony/etcd,jlamillan/etcd,jonboulle/etcd,westhood/etcd,westhood/etcd,handsomesun/etcd,glycerine/etcd,nekto0n/etcd,ajityagaty/etcd,ajityagaty/etcd,akauppi/etcd,vimalk78/etcd,ktateish/etcd,li-ang/etcd,linuxcer/etcd,YuleiXiao/etcd,etcd-io/etcd,sebrandon1/etcd,hongchaodeng/etcd,swingbach/etcd,hongchaodeng/etcd,jonboulle/etcd,insequent/etcd,CrazyJvm/etcd,linuxcer/etcd,rnaveiras/etcd